It is well known that Ultraviolet (UV) light can cause serious flash burns to the cornea from high intensity light sources. Thus our eyes need protection from these harmful UV lights. The situations where our eyes definitely need protection from UV lights are welding, exposure to sunlight at elevation above 5000 ft (1524 m), or when the sun glares off snow or water, tanning, etc.
Not just UV light, infrared is also harmful. Wireless communication, appliances, computer, and lights all emit different levels of harmful radiation. In fact, there are also plenty of natural infrared, such as those from sunlight. Sunlight is composed of thermal-spectrum radiation that is slightly more than half infrared. At zenith, sunlight provides an irradiance of approximately 1 kilowatt per square meter at sea level, of which 527 watts is infrared radiation. Once the sunlight reaches the surface of Earth, almost all thermal radiation are of infrared.
The energy of sunlight on the ground can be categorized into approximately 3% Ultraviolet (UV) rays, 44% visible rays, and 53% Infrared (IR) rays. Therefore, when exposed to intense sunlight for a lengthy period of time without protection, human eyes may experience a burning or stinging sensation that is often accompanied by fatigue. Such discomfort can be especially noticeable for those wearing contact lenses, as the infrared can be absorbed by the contact lenses causing them to “warm up”. Eye doctors always encourage a habit of putting on sunglasses when staying out in the sun for a period.
Traditionally, for protection lens to block harmful rays from a light source, the lenses must be coated with one or more layers of IR and/or visible dyes. Usually, soluble dyes and/or metallic oxide pigments are used for coating to absorb or reflect light of certain frequencies, eg., IR frequencies, UV frequencies, etc. Thus, coated lens would reduce or mitigate eye diseases such as cataract and glaucoma.
Because of the importance of sunglasses and protecting eyewear, there have been many coating techniques invented. IR or visible coating can be applied by dipping or spraying a solvent IR or visible dyes on another optical layer of a lens. However, because the majority of lenses are curved, the curvature of the lenses presents a significant obstacle in the application of the IR or visible coating, as the application of the coating may be uneven. As a consequence, the uneven application of the coating on a curved surface would reduce the effectiveness of the protection layers.
Using traditional methods like extrusion or injection, IR or visible dyes are added during the processes. Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or pulled through a die of the desired cross-section. In a plastic extruding process, plastic is first melted into a viscous, semi-liquid state. After it softens, the plastic is pressed through a contoured opening. Using this technique, a curved lens may be created by pushing a softened optical film through a contoured opening.
Injection molding is a manufacturing process for producing parts by injecting material into a mold. Material for the part is fed into a heated barrel, mixed, and forced into a mold cavity, where it cools and hardens to the configuration of the cavity. For optical plastic films, whether it is an extrusion or injection method, heat is needed to soften the plastic films so they can be shaped curvaceously. Since dyes are sensitive to heat, some dye degradation occurs, and the effectiveness of eye protection reduces.
Another problem with these IR or visible coated lens is that they are easily scratched and are not resistant to chemicals or elements. Over time the protection layers lose their effectiveness and become harmful if not detected and replaced. To overcome this problem lens manufacturers have put another protection layer on top of the IR/visible layer either by spraying, dipping, or injection. However, as a consequence, additional layers make the lens thicker and to have a minimum thickness, which is a barrier for eyewear design and comfort.
Furthermore, traditional coating methods by injection or extrusion methods are aesthetically less appealing because infrared dye appears green in such a coating. In order to counteract or offset the undesirable green color, gray colors may be added to the PVA film. The addition of such gray colors, however, reduces the penetration of light, and therefore the visibility for the viewers, significantly. Finally, the addition of the gray colors to the PVA films on the lens results in higher costs for the lenses, and thus higher costs for the end products. Therefore, material and manufacturing processes for IR absorbing lenses that are inexpensive and quick are desirable.
Recently, to overcome the drawbacks of extrusion and injection methods, solution casting method has been invented and preferred. This manufacturing technology is unique in that the process does not require conventional extrusion or injection molding technologies, yet it readily incorporates components and features traditionally produced by these processes. This method utilizes a mandrel, or inner diameter mold, that is immersed in a tank of polymer solution or liquid plastic that has been specifically engineered for the process. Due to a combination of thermal and frictional properties, the polymer solution then forms a thin film around the mold. The mold is then extracted from the tank in a precisely controlled manner, followed by a curing or drying process.
Other casting devices being used in a solution casting method are a belt or drum machines. Typically, supporting belts are 1.0 to 2.0 m wide and 10 to 100 m long. Stainless steel belts are between 1.0 and 2.0 mm thick. Drums are typically 4 to 8 m in diameter and 1.20 to 1.50 m wide. The belt channel allows a stream of air to flow in machine direction or counter direction. The drum is tightly sealed to prevent vapor emissions and to direct the air stream against the direction of drum movement. One of the two pulleys or drums is connected to a drive that requires extremely accurate speed control to avoid even slight speed variations. One drum is connected to a servo system that adjusts belt tension in order to ensure constant flatness and “absence” of belt movements (vibrations) in the critical area just behind the caster, and to control the expansion and dilatation of the belt length caused by temperature changes. Belt machines have a guide system to avoid belt shifting during operation. The belt is guided by horizontal movements of the support drums. Many different support materials have been used for belts: Copper, silver-plated copper, chromium-plated steel, stainless steel, metal coated with polyvinylalcohol or gelatin, polyester film, PTFE film and other polymer films. At present the commonest support materials are stainless steel and chromium-plated surfaces. Important items for belt and drum machines are the material's heat conductivity, the technical processes used to create the required surface finish and the options for repairing small surface defects. This cast technique permits simple production of films with structured surfaces. The belt surface is clearly and accurately replicated on one surface of the film. The techniques used to adapt the surface of the drums or belts to highly glossy, structured or matt film finishes are proprietary methods.
Once the first layer of thin film is appropriately solidified, secondary features can be added to the product such as braided or coiled wire, laser-cut hypotubes or engineered metal reinforcements to prevent kinking, or imaging targets specific to the intended medical application. Multiple casting steps can then be repeated to encapsulate the reinforcements, build up wall thickness, add additional lumens and optimize column strength. The part is then removed from the mold after it is cured or solidified. This method works with liquid forms of solvent polymers without using excessive heat to cure the part. Since this method uses centrifugal force to shape the part, with the right liquidity ratio, a very thin layer of IR or visible dye solution can be added to an optical film without using excessive heat.
Another method to make the film is a static method such as cavity mold or plate casting or other similar method.